With direct vent gas fireplaces, it is preferred to minimize the clearance required between the unit and combustible materials. The ideal is "zero clearance". To achieve this, fireplaces have been constructed with wrappings of thermal insulation and plural circulation chambers around the firebox. The air chambers heat room air as well as providing insulation. This increases the overall size of the fireplace, making the unit project some distance into a room when installed in a standard stud wall.
The applicant's Canadian patent application 2,205,242, filed May, 13, 1997 discloses a direct vent gas fireplace with an outer sheet metal housing a vertical outer back wall and a sloped outer top wall sloping upwardly from the outer back wall towards a front side of the housing. An inner sheet metal fire box is located inside the outer housing and has a vertical inner back wall spaced from the outer back wall of the housing. There is a combustion air inlet through the inner vertical back wall, adjacent its bottom end. A sloped inner top wall slopes upwardly from the inner back wall toward a front side of the fire box and is spaced from the outer top wall. A combustion air supply includes a plenum on top of the outer top wall and a duct extending from the plenum between the outer and inner back walls to the combustion air inlet. Two cooling air ducts are located between the inner and outer back walls on opposite sides of the combustion air duct and have inlets between the bottom of the outer housing and the inner bottom wall, and adjacent the respective side walls.
The combustion air duct and the cooling air ducts are thus arranged side by side in the back wall in order to reduce the depth of the unit. The cooling air is drawn from the bottom outside corners of the unit, below the firebox. The air in this area is the coolest available for supply to the cooling air ducts. This arrangement allows a reduction in the back wall thickness, while maintaining adequately low temperature levels on the outer back wall, even without a layer of thermal insulation in or on the back and top walls.
The present invention is concerned with certain improvements in direct vent gas fireplaces of this type.